CALVIN ON DARK SHALE AND ITS FOSSILS. 727 



It is an interesting fact that of the twelve determinable species six 

 occur only in the shaly deposits at the opening and close of the Devo- 

 nian, notwithstanding these deposits are separated by 150 feet of lime- 

 stone. Only one species is known to pass from the lower shales into the 

 limestones above, and even there it appears under a form so altered that 

 specimens from the two beds may be distinguished as readily as if they 

 were distinct species. If we take form and surface-markings into 

 account, the Atrypa reticularis of No. 1 also finds its nearest representa- 

 tive, not in the limestones immediately above, but in the shales at 

 Eockford. 



Obviously, then, the Independence shales are more nearly related to 

 the Eockford beds than to any other formation in Iowa. The species 

 in Group I seem to have disappeared with the ushering-in of condi- 

 tions under which limestones were formed ; they maintained themselves 

 in some locality which has not been discovered, or from which the shaly 

 deposits have been entirely swept away, and returned with the return 

 of conditions favorable to their existence during the deposition of the 

 Eockford shales. 



The intimate relation between the two extremes of the group is cer- 

 tainly a most interesting one, and can but strengthen the conclusion 

 of Dr. White, that all the Devonian strata of Iowa belong to a single 

 epoch. 



BracMopods of the Independence Shale. 



STROPHODONTA VARIABILIS, U. S. 



Shell small, very variable, thin, orbicular to semi-oval in outline. 

 Valves in some instances about equally convex, in other cases, notably 

 in young specimens, the dorsal valve has the greater convexity, the 

 ventral being iiat or even slightly concave; again the ventral valve 

 may be regularly convex, the dorsal being concave, or the dorsal valve 

 may be convex near the back, becoming deeply concave toward the 

 front margin. 



Hinge line straight; cardinal extremities often produced, but more 

 frequently rounded in adult individuals. Hinge-area common to both 

 valves, narrow, a little wider on the ventral, marked by a few strong 

 vertical strice corresponding to the deep crenulations of the hinge-line. 



Surface marked by fine radiating and alternating stride, -which are 

 strongly 'curved on the cardino-lateral areas and increase by implanta- 

 tion. Fascicles, of from 4 to 7 minute, low, rounded strice, occur between 

 each i^air of larger, angular, and much more prominent ones. An im- 

 perfectly defined mesial fold sometimes seen on ventral valve. Striae 

 crossed by very minute, microscopic, concentric lines. 



Muscular scars of ventral valve broad, short, and sharply defined by 

 an elevated ridge. Cardinal process of dorsal valve bifid, the diverging 

 parts slender, emarginate at tip, and fitting into notches in area of 

 ventral valve. Entire inner surface granulose. Vascular markings ob^ 

 scure, except near the margin. 



